1. Field of the Invention
The present invention relates to an arrayed waveguide grating (AWG) type optical multiplexer/demultiplexer which is employable as a wavelength selecting device in a wavelength division multiplexing (WDM) transmission system.
2. Related Background Art
AWG type optical multiplexer/demultiplexers (hereinafter referred to as AWG circuits) are widely in use as a wavelength filter, which can take out or insert a specific wavelength upon interference, for a wavelength-selecting device in WDM transmission systems. Also, since the AWG circuits can be realized by general fine processing procedures such as lithography or etching without necessitating the machining as precise as that of diffraction gratings or the forming of multilayer films as precise as that of interference films, they are expected to develop as a main optical device in future WDM transmission systems together with their capability of assembling with other optical waveguide devices.
Such an AWG circuit has a structure in which an input waveguide, an input slab waveguide, a plurality of channel waveguides having respective lengths different from each other (phased array), an output slab waveguide, and an output waveguide are integrally formed on a single substrate and are covered with cladding glass. For lowering loss in a conventional AWG circuit, in particular, it is necessary for the channel waveguides to be processed such that each has a rectangular cross-sectional structure, and that they are disposed closer to each other. Between adjacent waveguides in which the waveguides are disposed closer to each other as in a connecting portion between the slab and channel waveguides, however, a void may occur without being filled with cladding glass, whereby the AWG circuit may not be made with a favorable reproducibility as designed.
In order to prevent the incomplete filling of cladding glass from occurring as mentioned above and yield an AWG circuit which is easy to make with a favorable reproducibility, Japanese Patent Application Laid-Open No. HEI 7-63934 discloses a structure in which adjacent waveguides such as those between slab and channel waveguides are separated from each other. by about 1 to 10 xcexcm, whereas thus formed gap is filled with cladding glass.
The inventors have studied the conventional AWG circuits and, as a result, have found a problem as follows.
In the AWG circuit disclosed in Japanese Patent Application Laid-Open No. HEI 7-63934, adjacent waveguides are separated from each other by about 1 to 10 xcexcm in order to suppress the excess loss caused by diffraction to a maximum of 0.1 dB which is fully negligible. However, the conventional AWG circuits have been problematic in that crosstalk characteristics between adjacent signal channels remarkably deteriorate even when the space between the adjacent waveguides is set such that the excess loss caused upon filling with cladding glass can be suppressed to a fully negligible level.
In order to overcome the problem mentioned above, it is an object of the present invention to provide an optical multiplexer/demultiplexer which ameliorates the deterioration in crosstalk characteristics between adjacent channels in a simpler configuration with a better reproducibility.
The optical multiplexer/demultiplexer according to the present invention is an AWG type optical multiplexer/demultiplexer, employable as a wavelength-selecting device in a WDM transmission system, comprising a substrate, and at least one input waveguide, a first slab waveguide, a plurality of channel waveguides, a second slab waveguide, and a plurality of output waveguides provided for respective signal channels, which are disposed on the substrate.
In the optical multiplexer/demultiplexer according to the present invention, the first and second slab waveguides have respective predetermined slab lengths. In general, a slab length corresponds to the focal length of the optical input end functioning as the lens surface of the respective slab waveguide. The input waveguide is a waveguide for guiding to the first slab waveguide individual signals having respective channel wavelengths set at predetermined wavelength intervals as signal channels, and has an output end optically connected to an optical input end face of the first slab waveguide. The plurality of channel waveguides are waveguides having lengths different from each other , and are two-dimensionally arranged on the substrate while in a state where an optical input end of each channel waveguide is optically connected to an optical output end face of the first slab waveguide so as to sandwich the first slab waveguide together with the input waveguide whereas an optical out put end of each channel waveguide is optically connected to an optical input end face of the second slab waveguide so as to sandwich the second slab waveguide together with the output waveguides. The output waveguides are waveguides two-dimensionally arranged on the substrate while in a state where respective optical input ends thereof are optically connected to an optical output end face of the second slab waveguide, and are used for separately taking out signals having respective channel wavelengths set at predetermined wavelength intervals.
In particular, the inventors have found the fact that the deterioration in crosstalk between adjacent signal channels is remarkably ameliorated when adjacent waveguides are separated from each other by a predetermined value or more in a portion where the waveguides are disposed closer to each other, e.g., between the slab and channel waveguides, thereby accomplishing the optical multiplexer/demultiplexer according to the present invention.
Namely, in order to ameliorate the deterioration in crosstalk between adjacent signal channels caused upon separating adjacent waveguides from each other, at least one of the space between the optical input end of each channel waveguide and the optical output end face of the first slab waveguide, and the space between the optical output end of each channel waveguide and the optical input end face of the second slab waveguide is set to at least three times the width or thickness of each channel waveguide in the optical multiplexer/demultiplexer according to the present invention.
Preferably, at least the channel waveguides have a relative refractive index difference of 0.75% or more with respect to the substrate. It is because of the fact that if the relative refractive index difference of the channel waveguides with respect to the substrate is made greater, then the light confining effect improves, whereby the channel waveguide intervals can be set narrower.
Preferably, in the optical multiplexer/demultiplexer according to the present invention, the channel waveguides are arranged such that the respective optical input ends thereof oppose the optical output end face of the first slab waveguide over 90% or more of the area of the optical output end face in a direction perpendicular to the substrate. Preferably, the channel waveguides are arranged such that the respective optical output ends thereof oppose the optical input end face of the second slab waveguide over 90% or more of the area of optical input end face in a direction perpendicular to the substrate on the second slab waveguide side as well. It is because of the fact that a greater light capturing angle is further effective in ameliorating the crosstalk between adjacent signal channels.
The gap between the adjacent waveguides (e.g., slab and channel waveguides) somewhat fluctuates depending on the relative refractive index differences of these waveguides with respect to the substrate. Therefore, in order to ameliorate the deterioration in crosstalk between adjacent signal channels caused upon separating the adjacent waveguides from each other in the optical multiplexer/demultiplexer according to the present invention, it is further preferred that at least one of the space between the optical input end of each channel waveguide and the optical output end face of the first slab waveguide, and the space between the optical output end of each channel waveguide and the optical input end face of the second slab waveguide be set to 2M or more but 6M or less, where M is the mode field diameter of light propagating through the channel waveguide.
In order to improve the effect of buried cladding glass and ameliorate the crosstalk deterioration between adjacent signal channels in the optical multiplexer/demultiplexer according to the present invention, each input waveguide is preferably arranged on the substrate such that the optical output end thereof is separated from the optical input end face of the first slab waveguide by xc2xdor more of the thickness of the input waveguide. Similarly, each of the output waveguides is preferably arranged on the substrate such that the optical input end thereof is separated from the optical output end face of the second slab waveguide by xc2xdor more of the thickness of the output waveguide.
In a specific embodiment, the optical multiplexer/demultiplexer according to the present invention comprises thirty or more output waveguides (i.e., thirty or more signal channels to be multi/demultiplexed), and makes it possible to multi/demultiplex signal channels having a wavelength interval of 100 GHz or less. Therefore, in the optical multiplexer/demultiplexer, it is preferred that the channel waveguides be arranged at an interval of 15 xcexcm or less. Preferably, the output waveguides are arranged at an interval of 20 xcexcm or less. Preferably, each of the first and second slab waveguides is designed to have a slab length of 15 mm or less.